Over the past few years, asymmetric allyl-, crotyl-, and higher allyl boron reagents have proven to be exceptionally valuable in the context of acyclic stereoselection. Driven by the rapidly growing demand for highly enantiomerically pure substances in multi-step natural product syntheses, the development of superior allylborane reagents, which can achieve enantio- and diasterioselectivities approaching 100% has become both desirable and challenging.
A number of asymmetric allylboron reagents have been reported by various investigators. See Hoffmann, R. W. et al., Chem. Ber. 1981, 114, 375; Brown, H. C. et al., Am. Chem. Soc., 1983, 105, 2092; Idem. J. Org. Chem. 1984, 49, 4089; Roush, W. R. et al., Am. Chem. Soc. 1985, 107, 8786; Brown, H. C. et al., lbid. 1986, 108, 293; Rousch, W. R. et al., Ibid. 294; Brown, H. C. et al., J. Org. Chem. 1987, 52. 319; ldem. Ibid. 3701; Garcia, J. et al., Ibid. 4831; Brown, H. C. et al., S. J. Am. Chem. Soc. 1988, 110, 1535; Roush, W. R. et al., lbid. 3979; Short, R. P. et al, J. Am. Chem. Soc. 1989 111, 1982; and Corey, E. J. et al., J. Am. Chem. Soc. 1989, 5495. While asymmetric allylborations of aldehydes with comparatively high enantioselectivities has been achieved with prior art reagents, each of these reagents require the preparation of the chiral auxiliaries in several steps and some even require resolution.
The present invention provides a new class of reagents, B-(2-alkenyl)bis(2-isocaranyl)borane, which undergoes reaction with a variety of aldehydes to furnish the corresponding homoalkenylic alcohols in 94-99% ee. The enantioselectivities realized with this reagent are significantly higher than those realized previously with B-allyldiisopinocampheylborane and B-allylbis(4-ioscaranyl)borane (reported by Brown, H. C. et al, J. Org. Chem 1986 51, 432).